Printing quality examining method

ABSTRACT

A printing quality examining method in which image data printing paper is successively taken in by a camera of a detection unit. The image data are compared with a previously taken-in reference data to detect a printing defect so that a decision as to whether printing quality is good or bad is made. The image data is averaged in time to calculate an estimated data, and the estimated data is compared with the previously taken-in reference data. When the estimated data is normal, pixels are determined to be normal. When the estimated data is unusual, the image data from the detection unit is compared with the reference data at next step so that when the image data from the detection is normal, the pixels are determined to be normal and when the image data is unusual, the pixels are determined to be defective and the paper is discharged.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application of application Ser. No.08/595,994, filed Feb. 6, 1996 now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a printing quality examining apparatus(an apparatus for examining printing quality) used for decision as towhether printing quality is good or bad.

In a conventional printing quality examination method, as shown in FIG.10, image data of printing paper are successively read in order of k=1,. . . , k=n-1, k=n by means of a camera and the image data are averagedin time to obtain estimated value data. In FIG. 10, i, j and k are marksfor defining coordinates for picture or pattern data on paper. Theestimated value data is compared with a previously read reference dataand when a difference therebetween exceeds a threshold value, pixelsthereof are decided to be defective. A flow chart of this decisionoperation is shown in FIG. 11.

In the conventional printing quality examination method shown in FIGS.10 and 11, however, an estimated value data which is an average value ofimage data on current printing paper and image data on past printingpaper is compared with the reference data in order to improve detectionaccuracy. Accordingly, once any defect occurs, estimated value data ofnormal pixels on several printing papers used subsequently to theoccurrence of the defect are influenced by defective data in the past,so that the normal pixels are decided as defective pixels in error andconsequently even satisfactory paper is discharged as defective paper.

Further, FIG. 12 illustrates a printing quality examination apparatusused heretofore. This conventional printing quality examinationapparatus is now described with reference to FIG. 12. In FIG. 12,numeral 105 denotes a printing paper put on an impression cylinder A sothat the printing paper is curved arcuatedly, numeral 114 a cameradisposed in a detection unit, numeral 115 a xenon lamp disposed far fromthe detection unit, numeral 116 an optical fiber extended from the xenonlamp 115, numeral 117 a light irradiating end formed at an end of theoptical fiber 116, and numeral 118 an optical axis of the light source.The camera 114 is disposed so that an optical axis of the camera issubstantially vertical to the printing paper 105 and the lightirradiating end 117 is disposed so that an optical axis of the lightirradiating end 117 is oblique to the printing paper 105.

The printing paper 105 is irradiated by light from the light irradiatingend 117, while image data on the printing paper 105 is taken in by thecamera 114 and defective paper is detected from the image data on theprinting paper 105.

However, in the illumination apparatus of the conventional printingquality examination apparatus shown in FIG. 12, the printing paper 5 isilluminated by illumination light from the single light irradiating end117 and accordingly the xenon lamp 115 having the high luminousintensity is required. Furthermore, since the xenon lamp 115 is disposedfar from the detection unit and light from the xenon lamp is led to thedetection unit through the optical fiber 116, a manufacturing costthereof is increased.

Further, a condensing lens or the like is used to feed the illuminationlight from the xenon lamp 115 to the optical fiber 116 effectively. Thecondensing lens or the like must be maintained for the opticaldeterioration and a great deal of labor is required therefor.

In addition, at a held end of the printing paper 105 on the impressioncylinder A, when image data is taken in by the detection unit, an amountof light incident on the detection unit is varied due to fluttering ofthe paper, so that amendment of the illumination light amount isinfluenced greatly. More concrete description is given as follows.

When normal printing paper is irradiated by illumination lightvertically and an illuminometer 119 is moved in parallel to the printingpaper to measure the luminous intensity on the paper while theilluminometer is maintained vertically and in an equal distance to theprinting paper as shown in FIG. 13, the luminous intensity on the paperis expressed by a curve named a substantially normal distribution andthe distribution of the luminous intensity is maximized in the vicinityof the optical axis 120 of the illumination.

In the illumination apparatus of the conventional printing qualityexamination apparatus, as shown in FIG. 14, illumination light isincident along an incident axis 121 oblique to the vertical axis andpassing through an incident point of illumination light and the light isreflected along an emitting axis 122 having the same angle in theopposite direction. Accordingly, the maximum point of the luminousintensity on the paper is offset on the side of the reflected light withrespect to the vertical axis as shown by a curve of FIG. 14.

FIG. 15 illustrates variation in the luminous intensity on the printingpaper 105 due to a fluttering of paper occurring at the held end of theprinting paper 105 on the impression cylinder A.

When fluttering of paper does not occur at the held end of the printingpaper 105, the distribution of the luminous intensity shown by solidline is obtained and the luminous intensity on the paper on the opticalaxis of the camera 114 is I₀. The case where the held end of the paperis moved up is now considered. An amount of variation or movement isregarded to be able to be neglected as compared with a distance from thelight irradiating end 117 and the camera 114 to the printing paper 105and the printing paper 105 is assumed to be angularly moved or rotatedabout an intersection point O of the printing paper and the verticalline drawn from the camera 114 to the printing paper. The upwardmovement of the held end of the paper corresponds to the angularmovement of the printing paper 105 in the counterclockwise direction byan angle α.

At this time, the normal line extending from the intersecting point 0vertically to the printing paper 105 is rotated by α in thecounterclockwise direction similarly. Further, the emitting axis of thereflected light is rotated by 2α and the maximum point of the luminousintensity in the distribution of the luminous intensity on the printingpaper 105 is also moved leftward as shown by broken line in FIG. 15.Thus, an amount of light received by the camera 114 is varied from I₀ toI₁. This corresponds to a variation in the amount of light received bythe camera 114 due to fluttering of paper.

Further, FIG. 16 schematically illustrates a circuit configuration of aconventional printing quality examination apparatus. In this case, animage of a printing paper having the luminous intensity on the surfacethereof maintained constant by illumination light from an illuminatinglight source is taken in by a line camera of a detection unit 201. Theluminous intensity on the printing paper is maintained to be constantand decision as to whether the printing paper is good or bad is made asfollows.

As shown in FIG. 17, an output signal from the detection unit 201 isreduced in substantially inverse proportion to a machine speed. Theoutput signal from the detection unit 201 is supplied to an amplifier202 of FIG. 16 to be amplified and the amplified signal is supplied toan analog-to-digital (A/D) converter 203 to be converted to a digitalsignal. Thereafter, the digital signal is subjected to a correctionprocess with respect to the machine speed and is held to be a fixedsignal level as shown in FIG. 17. Then, the digital signal is suppliedto a comparison operation circuit 204 of FIG. 16 to be compared with apreviously taken-in reference image data. This operation result issupplied to a decision circuit 205 in which decision as to whether theprinting paper is good or bad is made.

The decision result is supplied to the control and display unit 206 inwhich unsatisfactory paper is discharged and an alarm to an operator isdisplayed.

However, the printing quality examination apparatus shown in FIGS. 16and 17 has the following problems.

The luminous intensity on the printing paper is maintained to beconstant by illumination light from the illumination light source andsince an amount of light received by the printing paper is reduced whenthe machine speed is increased, a gain is applied to the whole signal inorder to maintain the received light amount constant. Accordingly, thereare the following problems.

(1) Since a gain is applied to the whole signal, even noise isamplified.

(2) Variation of the received light amount depending on quality of papercannot be understood.

(3) Since an amount of light is reduced due to a life of theillumination light source, it is necessary to maintain the illuminationlight source.

Further, in the conventional printing quality examination apparatus, asshown in FIG. 18, an image data of a blank portion 311 of the printingpaper is taken in by the camera of the detection unit and brightness ofthis portion is compared with a previously set reference value of theluminous intensity. An amount of light of the illumination light sourceis corrected on the basis of the result thereof to thereby obtain theluminous intensity required in the detection unit.

However, in the conventional printing quality examination apparatusshown in FIG. 18, it is presupposed that the blank portion 311 forcorrection of the light amount is present in the printing paper.However, some printing papers have no blank portion and in this case, itis impossible to correct the light amount.

On the other hand, when the blank portion 311 necessary for thecorrection of the light amount exists in the vicinity of a pictureportion 312 and the picture portion is moved or shifted due tomechanical vibration or the like, an amount of light in the printingportion other than the blank portion 311 is sometimes detected. When thelight amount is corrected in such a state, there is a problem that acorrected light amount in the detection portion becomes instable.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above variousproblems and objects thereof are as follows:

It is a first object of the present invention to provide a printingquality examining method capable of eliminating a defect that somesatisfactory papers subsequent to defective paper are decided asdefective papers.

Further, it is a second object of the present invention to provide anillumination apparatus of a printing quality examining apparatus capableof (1) improving defect detection accuracy of printing paper extremely,(2) reducing a manufacturing cost and reducing labors required tomaintain optical parts.

In addition, it is a third object of the present invention to provide aprinting quality examining apparatus capable of (1) improving defectdetection accuracy of printing paper extremely, (2) detecting defect ofprinting paper with high stability, and (3) reducing labors required tomaintain an illumination light source.

Furthermore, it is a fourth object of the present invention to provide aprinting quality examining apparatus capable of (1) performingcorrection of an amount of light to even printing paper having no blankportion, (2) stabilizing a corrected light amount in a detectionportion, and (3) reducing a cost of printing paper when the paper isexpensive.

In order to achieve the first object, according to the presentinvention, in the printing quality examining method in which image dataof a printing paper successively taken in by a camera of a detectionunit is compared with a previously taken-in reference data to detect aprinting defect and decide whether the printing quality is good or bad,the image data of the printing paper taken in by the camera of thedetection unit is averaged in time to calculate estimated data and theestimated data is compared with previously taken-in reference data. Whenthe estimated data is normal, pixels are decided to be normal and whenthe estimated data is unusual, the image data from the detection unit iscompared with the reference data at the next step. When the image datafrom the detection unit is normal, the pixels are decided to be normaland when the image data is unusual, the pixels are decide to bedefective and the paper is discharged.

In a preferred aspect of the present invention, the estimated data andthe image data having characteristics complementary to each other arecompared with the reference data and decision as to whether the printingquality is good or bad is made on the basis of the resultant logicproduct.

According to the present invention with the above structure, only theunusual printing paper is decided to be unusual and there is no casewhere some satisfactory papers subsequent to defective paper are decidedto be defective notwithstanding satisfactory paper.

Further, in order to achieve the second object, according to the presentinvention, in the printing quality examination apparatus in which adetection unit including illumination light sources and a camera isdisposed in opposite to an arcuatedly curved printing paper and adefective paper is detected from image data of the printing paper takenin by the detection unit, there is provided two illumination lightsources and the two illumination light sources are disposed so that axesof illumination light from the two illumination light sources aresymmetrically oblique to an optical axis of the camera and have equalangles with respect to the optical axis of the camera.

With the above structure, in the printing quality examining apparatus ofthe present invention, when the printing paper is rotatedcounterclockwise by α, an emitting axis of one illumination light sourceis also rotated in the same direction by 2α. Accordingly, the maximumpoint of the distribution of luminous intensity on the paper by the oneillumination light source is moved leftward and an amount of light onthe optical axis of the camera is increased in the same manner as theprior art. However, the maximum point of the distribution of luminousintensity by the other illumination light source is also moved leftwardto thereby reduce the amount of light on the optical axis of the camera.In this manner, since the light amounts of both the illumination lightsources are increased and reduced relatively, the illumination apparatusalways emits a fixed amount of light with respect to the camera even ifthe printing paper is varied due to fluttering of the held end thereof.

Furthermore, in order to achieve the third object, according to thepresent invention, in the printing quality examining apparatus includinga line camera of a detection unit for taking in image data of printingpaper, an illumination light source for illuminating the printing paperto ensure an amount of light required by the line camera, a cameracontroller for controlling the line camera, and an examinationcontroller for comparing the image data obtained by the line camera withreference image data to decide whether the quality of the printing paperis good or bad, the printing quality examining apparatus comprises alight amount correction unit for varying an amount of light of theillumination light source in accordance with printing speed, paperquality, the light source and the like and holding a received lightamount of the line camera to be constant, so that a signal level of theimage data is corrected.

In a preferred aspect of the present invention, the light amountcorrecting unit comprises a light amount difference detection unit forcomparing image data of a blank portion of the printing paper with apreviously set target luminous intensity and a light amount differencecorrection circuit for obtaining a correction value for a difference ofthe luminous intensity from the light amount difference detection unit.

According to the present invention with the above structure, the blankportion of the printing paper is read by the line camera of thedetection unit and an image data of the blank portion is supplied to thelight amount correction unit in which the image data of the blankportion is compared with the previously set target luminous intensity toobtain the difference of light amount. A correction voltage forcorrecting a voltage value of a power supply for the light source isobtained while using the difference of light amount as a parameter. Thepower supply for the light source is controlled by the correctionvoltage to change the luminous intensity of the illumination lightsource, so that a signal having a fixed level is always obtained fromthe detection unit without relation to the machine speed. That is, whenthe machine speed is increased and a received light amount of thedetection unit is reduced, the illumination light source is controlledso that the illumination light amount can be increased correspondinglyto maintain the received light amount from the blank portion of thedetection unit to be always constant.

In addition, in order to achieve the fourth object, according to thepresent invention, in the printing quality examining apparatus in whichprinting paper is illuminated by an illumination light source and imagedata of the printing paper is taken in by a camera of a detection unitto decide whether the printing quality is good or bad in accordance withthe image data, the printing quality examining apparatus comprises amaximum light amount pixel detection circuit for selecting a pluralityof maximum light amount pixels from image data taken in by the cameraand determining and holding pixel positions thereof, a light amountaveraging circuit for calculating an average value of the maximum lightamount pixels, and a difference circuit for comparing the average valuewith a previously set reference value of light amount to calculate adifference thereof. Further, in the present invention, when the printingpaper is illuminated by the illumination light source and the image dataof the printing paper is taken in by the camera of the detection unit todecide whether the printing quality is good or bad on the basis of theimage data, the plurality of maximum light amount pixels are selectedfrom the taken-in image data by the maximum light amount pixel detectioncircuit and the pixel positions thereof are determined and held. Theaverage value of the maximum light amount pixels is calculated by thelight amount averaging circuit and the average value is compared withthe previously set reference value of light amount by the differencecircuit to calculate a difference thereof. A voltage variation iscalculated while using the difference as a parameter and the lightamount of the illumination light source is corrected in accordance withthe voltage variation to obtain the optimum illumination condition bythe detection unit.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram schematically illuminating the wholeconfiguration of a printing quality examining apparatus according to afirst embodiment of the present invention;

FIG. 2 is a flow chart showing decision operation of the printingquality examining apparatus of the present invention;

FIG. 3 is a side view showing an embodiment of an illumination device(detection unit) of the printing quality examining apparatus accordingto the present invention;

FIG. 4 is a diagram explaining operation of the illumination apparatusshown in FIG. 3;

FIG. 5 is a circuit diagram schematically illustrating a light amountcorrection unit in a first embodiment of the printing quality examiningapparatus according to the first embodiment of the present invention;

FIG. 6 is a diagram illustrating structure of image data in the printingquality examining apparatus according to the first embodiment of thepresent invention;

FIG. 7 is a schematic diagram illustrating a printing quality examiningapparatus according to a second embodiment of the present invention;

FIG. 8 is a schematic diagram illustrating a portion of the printingquality examining apparatus shown in FIG. 7;

FIG. 9 is a diagram showing a relation of an amount of illuminationlight and a luminous intensity on paper in the printing qualityexamining apparatus shown in FIG. 7;

FIG. 10 is a diagram illustrating a conventional printing qualityexamining method;

FIG. 11 is a flow chart showing decision operation of the printingquality examining method shown in FIG. 10;

FIG. 12 is a side view illustrating an illumination apparatus of theconventional printing quality examining apparatus;

FIG. 13 is a diagram illustrating a distribution of luminous intensityof the illumination apparatus shown in FIG. 12;

FIG. 14 is a diagram illustrating a distribution of luminous intensityon paper of the illumination apparatus shown in FIG. 12;

FIG. 15 is a diagram illustrating operation of the illuminationapparatus shown in FIG. 12;

FIG. 16 is a schematic diagram illustrating a conventional printingquality examining apparatus;

FIG. 17 is a diagram showing a relation of the machine speed, acorrected output signal and a noise level of the printing qualityexamining apparatus shown in FIG. 16; and

FIG. 18 is a plan view showing printing paper upon examination of theprinting quality in a prior art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIGS. 1 to 9, first and second embodiments of thepresent invention are described in detail.

FIG. 1 schematically illustrates a portion of the whole configuration ofa printing quality examining apparatus according to a first embodimentof the present invention and FIG. 2 is a flow chart showing the printingquality examining method performed by the apparatus.

Before the whole configuration of the printing quality examiningapparatus is described, the basic function of the printing qualityexamining apparatus according to the present invention is firstdescribed with reference to the flow chart of the decision operationshown in FIG. 2.

Image data of printing paper are taken in by a camera of a detectionunit successively and the successively taken-in image data are averagedin time to calculate an estimated data. Then, the estimated data iscompared with a previously taken-in reference data. When the estimateddata is normal, pixels are decided to be normal and when the estimateddata is unusual, the image data from the detection unit is compared withthe reference data at next step. When the image data from the detectionunit is normal, the pixels are decided to be normal and when the imagedata is unusual, the pixels are decided to be defective and the printingpaper is discharged.

At first, when a printing condition is stabilized after a lapse ofcertain time since the printing has been started, the operator makes adecision from the printing paper and reads a reference picture data intothe memory.

In this case, the printing material is separated into pixels of1,000×1,000 and is stored in the memory. Then, an inspection permissiblevalue is calculated for each pixel of the reference picture data, and,based on this inspection permissible value, a decision is made whetherthe estimated data is normal or not.

A decision of whether the expression (model expression) of theinspection permissible value for each pixel is normal or abnormal ismade based on (reference data) ±(inspection permissible value):

    ε(S.sub.ij)=a×S.sub.ij ×b

where ε(S_(ij))=inspection permissible value, and

S_(ij) =data for each pixel of the reference picture.

More specifically, a decision is made that the model expression isabnormal when reference data-estimated>±inspection permissible value,and a decision is made that the model expression is normal whenreference data-estimated data≦±inspection permissible value.

The flow of the picture data to be read from the printed paper by acamera is as follows. (The reference picture data is stored in thememory by storing the data of a certain page when the printing has beenstabilized, as described above.)

1. Picture data of each pixel V_(ij) (k)

2. Take an average of this data and the estimated data of one page ofeach pixel P_(ij) (k-1), and calculate an estimated data P_(ij) (k) ofthis page.

3. Compare this P_(ij) (k) with the reference data S_(ij), and make adecision based on the result of the comparison. In other words, the datataken in by the camera is not directly compared with the reference data,but the raw data is processed (an average value, that is, an average intime direction) and the processed estimated data is compared with thereference data.

The expression for this processing is as follows:

    P.sub.ij (k)=a×{V.sub.ij (k)-P.sub.ij (k-1)}+P.sub.ij (k-1)}

where

P_(ij) (k)=estimated data

V_(ij) (k)=camera picture data

V_(ij) (k-1)=estimated data of one page before

i=coordinates in x direction

j=coordinates in y direction

k=number of pages in time direction

(k: this page; k-1: one page before; k-2: two pages before, and so on)

Accordingly, the data stored in the memory of the operating circuit areas follows:

1. Pixel data of 1,000 of 1,000 stored before starting the inspection.

2. Estimated data of 1,000 of 1,000 averaged in time direction based onthe picture data from the camera.

These two data are compared and the result is collated with theinspection permissible value shown in the previous discussion above.

Such an examination shown by the flow chart is performed in a decisioncircuit 24 of FIG. 1 or 7. That is, an algorithm of the aboveexamination is set in the decision circuit 24.

According to the printing quality examination apparatus of the presentinvention, the following effects can be attained. That is, since theestimated data is obtained by averaging the image data in time, highstability is obtained, whereas influence of defective image data in thepast is inherited. On the other hand, the image data taken in by thedetection unit has inferior stability but is not influenced by thedefective data in the past. Values having the complementarycharacteristic are compared with the reference data and decision as towhether the printing quality is good or bad is made so that a defectthat some satisfactory papers subsequent to defective paper are decidedas defective papers can be eliminated.

In the present system, there are the following three picture data:

1. Data V_(ij) (k) directly read from the camera.

2. P_(ij) (k) which is a result of time averaging with V_(ij) (k).

3. Reference data S_(ij).

The "complementary" data refers to (i) V_(ij) (k) and (ii) P_(ij) (k).

Referring to FIG. 2, a decision about the print quality is made usingthe picture data V_(ij) (k), as indicated at block 412. The estimateddata P_(ij) (k) is the result of time averaging with V_(ij) (k), asshown in block 414. The estimated data P_(ij) (k) is compared with thereference data S_(ij), as indicated in diamond 416. If the comparison is"normal", the pixel is satisfactory (block 418). Thus, if the estimateddata is normal, the image data of the printing paper successively takenin by the camera is averaged to calculate an estimated data which iscompared with previous taken in reference data. In the case where anestimated data is abnormal, a comparison is made between the picturedata and the reference data S_(ij) at diamond 420. If the comparison is"unusual", the pixel is defective (blocks 422, 424).

Thus, there are two decision paths: (1) a flow that a pixel is decidedto be normal if an estimated data is normal in comparison of theestimated data and the reference data with each other; and (2) a flowthat an estimated data is compared with the reference data in a step; ifthe estimated data is abnormal in the step, an image data is comparedwith the reference data in a next step; if an image data is normal inthe next step, a decision is made that a pixel is normal; but if theimage data is abnormal in the next step, a decision is made that a pixelis defective.

Thus, the term "complementary" refers to an estimated data and an imagedata associated with each other in a complementary relation or in arelation that both form a complete or better whole together. In a moredetailed manner, "complementary" then refers to a complementary relationbetween an estimated data processed by averaging the raw data thereoftaken in by a camera in time, which processed data is used as acomparative object in order to decide on whether a pixel is good or bad,and an image data. The estimated data is data which inherits aninfluence of a defective image data in the past, while a high stabilityis attainable. On the other hand, the image data is data which inheritsno influence of a defective image data in the past, while a highstability is not attainable.

From this standpoint, in the printing quality examining apparatus inwhich the image data of printing paper taken in by the camera of thedetection unit are compared with the previously taken-in reference datato detect printing defect so that decision as to whether the printingquality is good or bad is made, the image data of printing papersuccessively taken in by the camera of the detection unit are averagedin time to calculate an estimated data and the estimated data iscompared with the previously taken-in reference data. When the estimateddata is normal, pixels are decided to be normal and when the estimateddata is unusual, the imaged data from the detection unit are comparedwith the reference data is next step. When the image data from thedetection unit is normal, the pixels are decided to be normal and whenthe image data is unusual, the pixels are decided to be defective andthe printing paper is discharged. Accordingly, the defect that somesatisfactory papers subsequent to defective paper are decided to bedefective can be eliminated.

Referring now to FIGS. 1 to 6, the whole configuration of the printingquality examining apparatus according to the first embodiment of thepresent invention is described.

As shown in FIG. 1, the printing quality examination apparatus of theembodiment comprises a decision control unit A including a decisioncircuit 24 which performs the above-mentioned examination algorithm, adetection unit (illumination apparatus) B, and a light amount correctionunit C. The illumination apparatus of the printing quality examinationapparatus includes illumination light sources 1 and 2 and a camera 3,and the illumination light sources 1 and 2 and the camera 3 constitutethe detection unit B.

In FIG. 3, numeral 4 denotes an optical axis of the camera, 5 a printingpaper which is curved acruatedly on an impression cylinder A, 6 anincident axis of the light source 1, and 7 an incident axis of the lightsource 2. The detection unit B including the illumination light sources1 and 2 and the camera 3 is disposed in opposite to the printing paper 5curved arcuatedly on the impression cylinder A.

Further, the illumination light sources 1 and 2 are disposed so that theincident axis 6 of the illumination light source 1 and the incident axis7 of the illumination light source 2 are symmetrically oblique to theoptical axis of the camera 3 and have equal angles with respect to theoptical axis of the camera 3.

Operation of the illumination apparatus of the printing qualityapparatus shown in FIG. 1 is now described concretely with reference toFIG. 4.

When the printing paper 5 is rotated counterclockwise by α, an emitting8 of the illumination light source 1 is also rotated in the samedirection by 2α. Accordingly, the maximum point of the distribution 12of luminous intensity on the paper by the rotated illumination lightsource 1 is moved leftward and an amount of light on the optical axis ofthe camera is increased as in the prior art. However, the maximum pointof the distribution 13 of luminous intensity by the rotated illuminationlight source 2 is also moved leftward to thereby reduce the light amounton the optical axis of the camera. In this manner, the light amounts ofthe illumination light sources 1 and 2 are increased and reducedrelatively and accordingly the illumination apparatus always produces afixed light amount with respect to the camera 3 even if the printingpaper 5 is varied due to fluttering of the held end.

Numeral 8 denotes an emitting axis of the illumination light source 1, 9an emitting axis of the illumination light source 2, 10 the distributionof luminous intensity on paper by the illumination light source 1, and11 the distribution of luminous intensity on paper by the illuminationlight source 2.

In the illumination apparatus of the printing quality examiningapparatus of the present invention, when the printing paper is rotatedcounterclockwise by α, the emitting axis of one illumination lightsource is also rotated in the same direction by 2α. Accordingly, themaximum point of the distribution of luminous intensity on paper by oneillumination light source is moved leftward and the amount of light onthe optical axis of the camera is increased as in the prior art.However, the maximum point of the distribution of luminous intensity bythe other illumination light source is also moved leftward to therebyreduce the amount of light on the optical axis of the camera. In thismanner, since the light amounts of both the illumination light sourcesare increased and reduced relatively, the illumination apparatus canproduce a fixed amount of light with respect to the camera even if theprinting paper is varied due to fluttering of the held and thereof, sothat the detection accuracy of defects can be improved remarkably.

Further, an inexpensive fluorescent lamp can be used for theillumination light source. In addition, optical parts such as an opticalfiber and the like are not required, so that a manufacturing cost can bereduced and labor necessary for maintenance of optical components can bereduced.

The decision control unit A is a basic element which performs theprinting quality examination shown in FIG. 2 on the basis of thedetection signal from the detection unit B and, as shown in FIGS. 1 and5, includes the detection unit B having a line camera 3 or the like, ananalog-to-digital (A/D) converter 20 for converting the image datasignal from the detection unit B from an analog value to a digitalvalue, a light amount value averaging circuit 21 for averaging thedigitally converted image data from the A/D converter 20, a comparisonoperation circuit 23 for comparing the averaged image data produced bythe circuit 21 with reference image data (data from a reference dataunit 22), a decision circuit 24 for deciding whether the printingquality is good or bad on the basis of the comparison result from thecomparison operation circuit 23, and a control and display unit 25 forperforming control or display on the decision result from the decisioncircuit 24. There is a direct connection between the A/D converter 20and the comparison operation circuit 23, as shown in FIG. 1.

Further, the apparatus shown in FIG. 1 includes the light amountcorrection unit C in relation to the decision control unit A. As shownin FIGS. 1 and 5, the light amount correction unit C includes thedetection unit B having the line camera 3 or the like, an A/D converter20 for converting the image data signal taken in by the line camera 3 ofthe detection unit B from an analog value to a digital value, a maximumlight amount pixel position detection circuit 26 for selecting aplurality of maximum light amount pixels from the digitally convertedimage data signal from the A/D converter 20 to determine and hold pixelpositions thereof, a pixel data detection circuit 27 for detecting pixeldata (light amount) in the positions detected by the circuit 26, and apower supply 28 for the light source for supplying electric power basedon a control signal from the circuit 27 to the illumination lightsources 1 and 2. The pixel data (light amount) detection circuit 27includes, as shown in FIG. 5, a light amount averaging circuit 31 forcirculating an average value (current value) Lw of the maximum lightamount pixels produced from the maximum light amount pixel detectioncircuit 26 and a difference circuit 21 for comparing the average value(current value) Lw with a previously set reference value Lt of the lightamount to calculate a difference ΔL. In FIG. 5, the above A/D converter20 is omitted for simplification of the circuit.

Operation of the printing quality examining apparatus shown in FIG. 5 isnow described concretely.

Image data constituting a reference image is taken in by the camera 3 ofthe detection unit B at the time that the printing state is stable. Aplurality of pixels having the maximum brightness are selected from allpixels constituting the image data by the maximum light amount pixeldetection circuit 26. Further, pixels of a previously set number areselected from the pixels and are defined as noticeable pixels 35 (referto FIG. 6). In FIG. 6, numeral 36 denotes printed image data, 37 an areain which the noticeable pixel exists, and 38 peripheral pixels.

When the brightness value of the noticeable pixel 35 is compared withbrightness values of a plurality of pixels existing in the vicinity ofthe noticeable pixel (hereinafter referred to as peripheral pixels) andwhen differences thereof fall in a predetermined set threshold, thenoticeable pixel 35 is defined to be the maximum light amount pixel.However, when the differences of the brightness values do not fall inthe threshold, the noticeable pixel 35 is regarded as data containinginfluence such as noise and is not defined to be the maximum lightamount pixel. The position of the maximum light amount pixel thusdetermined is held.

In the examination of the printing paper, pixel data are successivelytaken in by the camera 3 of the detection unit B and pixelscorresponding to positions of the above maximum light amount pixels areselected from the pixel data. An average value Lw of brightness iscalculated from the maximum light amount pixels by the light amountaveraging circuit 30 and the average value Lw of brightness is suppliedto the difference circuit 31 as a current light amount. In thedifference circuit, the average value Lw of brightness is compared witha previously set light amount reference value (target value of lightamount) Lt to calculate a difference ΔL (that is, Lt-Lw). A voltagevariation ΔV is calculated while using the difference ΔL as a parameterand the power supply 28 for the light source is controlled by thevoltage variation ΔV to thereby correct the light amount of theillumination light sources 1 and 2 so that the optimum illuminationcondition is obtained by the detection unit B.

The reference data is usually taken in the reference data unit 22 onlyupon taking in of the reference data except the examination time.

In the light amount correction unit C provided in the printing qualityexamining apparatus of the present invention, when the printing paper isilluminated by the illumination light source and the image data of theprinting paper is taken in by the camera of the detection unit to decidewhether the printing quality is good or bad on the basis of the imagedata, a plurality of maximum light amount pixels are selected from thetaken-in image data by the maximum light amount pixel detection circuitand positions of these pixels are determined and held. An average valueof the maximum light amount pixels is calculated by the light amountaveraging circuit and is compared with the previously set light amountreference value to calculate a difference thereof by the differencecircuit. The voltage variation is calculated while using the differenceas a parameter and the light amount of the illumination light source iscorrected by the voltage variation to obtain the optimum illuminationcondition by the detection unit. Accordingly, the correction of thelight amount can be made for the printing paper having no blank portion.

Further, the light amount of a printed portion except the blank portionis not read and the corrected light amount in the detection unit can bestabilized.

Furthermore, in the examination of the printing quality, since the blankportion of the printing paper can be eliminated, a cost of printingpaper can be reduced when the printing paper is expensive.

FIGS. 7 to 9 illustrate a second embodiment of the present invention.The embodiment includes a light amount correction unit D shown in FIGS.7 and 8 instead of the above-mentioned light amount correction unit Cand is applicable to the case where the printing paper has a blankportion. In FIGS. 7 and 8, the same portions as those of FIG. 1 aredesignated by the same reference numerals and description thereof isomitted.

As shown in FIG. 7, the printing quality examination apparatus of theembodiment includes the light amount correction unit D in relation tothe decision control unit A as described above. The light amountcorrection unit D includes, as shown in FIGS. 7 and 8, a light amountdifference detection unit 40 for comparing the digitally converted imagedata (image data of blank portion) from the A/D converter 20 with thepreviously set target luminous intensity value (maximum light amountreference value), a light amount difference correction circuit 41 forcalculating a correction value for a difference of the luminousintensity values from the light amount difference detection unit 40, anda power supply 42 for light sources. As shown in FIG. 8, the lightamount difference detection unit 40 and the light amount differencecorrection unit 41 constitute a light amount correction unit 43.Electric power is supplied from the power supply for light sourceshaving a voltage which is varied to the illumination light sources 1 and2 by the light amount correction unit 43.

Operation of circuit of the printing quality examining apparatus shownin FIG. 7 is now described concretely.

A blank portion on the printing paper is read by the line camera of thedetection unit B and the image data of the blank portion is supplied tothe A/D converter 20 to be converted to a digital value. The digitallyconverted image data Lw of the blank portion is supplied to the lightamount difference detection unit 40 of the light amount correction unit43 and is compared with the previously set target luminous intensityvalue (maximum light amount reference value) Lt to obtain a differenceof light amount ΔL (Lt-Lw).

A correction voltage ΔV for correcting a voltage value of the powersupply 42 for light sources is obtained by the light amount differencecorrection circuit 41 while using the light amount difference ΔL (Lt-Lw)as a parameter and the power supply 42 for light sources is controlledto change the luminous intensity of the illumination light sources 1 and2 so that a signal having a fixed level is always obtained from thedetection unit B without relation to the machine speed. In other words,as shown in FIG. 9, when the machine speed is increased and the receivedlight amount of the detection unit B is reduced, the illumination lightsources 1 and 2 are controlled to increase the illumination light amountof the reduced amount so that the received light amount from the blankportion of the detection unit B is always maintained to be constant.

According to the light amount correction unit D of the printing qualitydetection apparatus, the blank portion of the printing paper is read bythe line camera of the detection unit B as described above and the imagedata of the blank portion is supplied to the light amount correctionunit in which the image data of the blank portion is compared with thepreviously set target luminous intensity to obtain the light amountdifference. The correction voltage for correcting the voltage value ofthe power supply for light sources is obtained while using the lightamount difference as a parameter. The power supply for light sources iscontrolled by the correction voltage to change the luminous intensity ofthe illumination light source so that the signal having a fixed level isalways obtained from the detection unit without relation to the machinespeed. In other words, when the machine speed is increased and thereceived light amount of the detection unit is reduced, the illuminationlight source is controlled to increase the illumination light amount bythe reduced amount so that the received light amount from the blankportion of the detection unit is always maintained to be constant.Accordingly, the amplifier 202 shown in FIG. 16 can be eliminated andthe problem that noise component produced in the amplifier 202 isamplified can be solved, so that the S/N (signal-to-noise) ratio of thesignal can be improved remarkably to thereby improve the defectdetection performance of the printing paper greatly.

Further, to always maintain the received light amount from the blankportion of the detection unit B constant is to always obtain the optimumreceived light amount by the detection unit B even for the printingquality having different reflectivity or brightness and even forvariation of the machine speed, so that defect of the printing paper canbe detected with high stability.

What is claimed is:
 1. A printing quality examining method in whichimage data of printing paper successively taken in by a camera of adetection unit are compared with a previously taken-in reference data todetect a printing defect so that a decision as to whether printingquality is good or bad is made, comprising averaging the image data ofthe printing paper successively taken in by the camera of the detectionunit in time to calculate an estimated data and comparing the estimateddata with the previously taken-in reference data so that when theestimated data is normal, pixels are determined to be normal and whenthe estimated data is unusual, the image data from the detection unit iscompared with the reference data at next step so that when the imagedata from the detection is normal, the pixels are determined to benormal and when the image data is unusual, the pixels are determined tobe defective and the paper is discharged.
 2. A printing qualityexamining method according to claim 1, wherein the detection unitincluding two illumination light sources and the camera is disposed inopposite to the printing paper curved arcuatedly and said twoillumination light sources are disposed so that respective optical axesof said two illumination light sources are symmetrically oblique to anoptical axis of the camera and have equal angles with respect to theoptical axis of the camera, defective paper being detected from theimage data of the printing paper taken in by the detection unit.